Fluid streams, such as water or air, often include contaminants like dissolved halogenated or organic compounds, volatile organic compounds, nitrogen oxides, inorganic gases like hydrogen cyanide, and microorganisms such as bacteria, viruses, molds, and fungi. Photocatalysts can be used to purify the fluid stream by converting these contaminants into less harmful substances or materials which may be more easily removed from the fluid stream.
The conversion of contaminants occurs when the fluid stream is brought in contact with a photocatalyst illuminated by a nearby light source. The photocatalyst is typically deposited on the surface of a support structure of some type to provide a stable photocatalytic surface and to ensure that the photocatalyst is not carried away by the fluid stream. Reactors employing these basic concepts have been developed. Other design factors, however, greatly influence reactor configuration.
To be effective, the contaminants must be brought into contact with the photocatalyst. The effectiveness of this process is measured by the mass transfer coefficient of the reactor which is the rate at which the contaminant is transported from the fluid stream to the photocatalytic surface. If the mass transfer system of the reactor is inadequate then conversion of contaminants will be diminished. Thus, an effective reactor design should provide for adequate mass transfer from the bulk fluid to the photocatalyst.
Another design consideration is the pressure drop in the reactor. Pressure drop occurs when objects interrupt the fluid flow thereby creating a pressure differential between the fluid on opposite sides of the hindering objects. To minimize pressure drop, fluid flow should be interrupted as little as possible to maintain a laminar flow. Significant pressure drop in the reactor will increase the operating costs of the system dramatically.
There is, however, a fundamental tension between design characteristics that increase mass transfer to the catalyst and those that decrease pressure drop. An increase in mass transfer typically requires increased contact with the support structure, whereas a decrease in pressure drop typically requires less interference by the support structure. Typical reactor designs place the support structures perpendicular to the fluid stream to ensure that the stream impacts the surface containing the photocatalyst to provide good mass transport. This reactor configuration, however, has a decidedly negative impact on pressure drop. Thus, there is a need for a reactor design which will provide adequate mass transfer to eliminate the contaminants in the fluid but at the same time provide a low pressure drop so that the fluid stream will flow smoothly through the reactor.
In addition, the photocatalyst must have sufficient contact time with the contaminant to catalyze the conversion reaction. The required contact time is determined by the kinetics of the catalyzed conversion reaction, taking into account competing reactions of other components in the fluid stream. The kinetics of a photocatalyzed reaction depend, in part, on the intensity of the light irradiating the photocatalyst. When the light intensity is too low the photocatalyst is not fully utilized and can not convert all of the contaminants that come in contact with it. Thus, an effective reactor design should provide for adequate illumination over the entire photocatalytically active surface.
Another desirable feature is the efficient use of illumination from the light sources within the reactor. Efficient illumination of a material or support with a large surface area per unit volume would provide the basis for a highly desirable compact reactor. In addition, a suitable reactor should be easy to maintain, manufacture, and service. The replacement of light sources and support structures should be easily accomplished with a minimum of difficulty.
These aspects of reactor design have not been sufficiently addressed in current designs. There is a need for a reactor with one and preferably more of the following characteristics: compactness, low pressure drop coupled with adequate mass transfer to the photocatalyst, efficient use of light to illuminate photocatalyst dispersed on a high surface area support, simple maintenance of lamps and photocatalyst support structures, and low cost manufacturing, maintenance and repair.